Marc Heyns

On the Correct Extraction of Interface Trap Density of MOS Devices With High-Mobility Semiconductor Substrates

ldquoConventionalrdquo techniques and related capacitance-voltage characteristic interpretation were established to evaluate interface trap density on Si substrates. We show that blindly applying these techniques on alternative substrates can lead to incorrect conclusions. It is possible to both under- and overestimate the interface trap density by more than an order of magnitude. Pitfalls jeopardizing capacitance-and conductance-voltage characteristic interpretation for alternative semiconductor MOS are elaborated. We show how the conductance method, the most reliable and widely used interface trap density extraction method for Si, can be adapted and made reliable for alternative semiconductors while maintaining its simplicity.

Soft breakdown of ultra-thin gate oxide layers

The dielectric breakdown of ultra-thin 3 nm and 4 nm SiO/sub 2/ layers used as a gate dielectric in poly-Si gate capacitors is investigated. The ultra-thin gate oxide reliability was determined using tunnel current injection stressing measurements. A soft breakdown mechanism is demonstrated for these ultra-thin gate oxide layers. The soft breakdown phenomenon corresponds with an anomalous increase of the stress induced leakage current and the occurrence of fluctuations in the current. The soft breakdown phenomenon is explained by the decrease of the applied power during the stressing for thinner oxides so that thermal effects are avoided during the breakdown of the ultra-thin oxide capacitor. It is proposed that multiple tunnelling via generated electron traps in the ultra-thin gate oxide layer is the physical mechanism of the electron transport after soft breakdown. The statistical distributions of the charge to dielectric breakdown and to soft breakdown for a constant current stress of the ultra-thin oxides are compared. It is shown that for accurate ultra-thin gate oxide reliability measurements it is necessary to take the soft breakdown phenomenon into account.

Trap-assisted tunneling in high permittivity gate dielectric stacks

The electrical characteristics of SiOx/ZrO2 and SiOx/Ta2O5 gate dielectric stacks are investigated. The current–density JG in these dielectric stacks is shown to be strongly temperature dependent at low voltage (below about 2 V), the more so in the ZrO2 stack. On the other hand, JG is much less temperature dependent at higher voltage. These results are consistent with a model which takes into account the direct tunneling of electrons across the SiOx layer and the trap-assisted tunneling of electrons through traps with energy levels below the conduction band of the high permittivity dielectric layer. The energy levels and densities of these electron trapping centers are estimated by fitting this trap-assisted tunneling model to the experimental results.

Effective electrical passivation of Ge(100) for high-k gate dielectric layers using germanium oxide

In search of a proper passivation for high-k Ge metal-oxide-semiconductor devices, the authors have deposited high-k dielectric layers on GeO2, grown at 350–450°C in O2. ZrO2, HfO2, and Al2O3 were deposited by atomic layer deposition (ALD). GeO2 and ZrO2 or HfO2 intermix during ALD, together with partial reduction of Ge4+. Almost no intermixing or reduction occurs during Al2O3 ALD. Capacitors show well-behaved capacitance-voltage characteristics on both n- and p-Ge, indicating efficient passivation of the Ge∕GeOx interface. The density of interface states is typically in the low to mid-1011cm−2eV−1 range, approaching state-of-the-art Si∕HfO2∕matal gate devices.

Determination of tunnelling parameters in ultra-thin oxide layer poly-Si/SiO2/Si structures

Abstract In this work the electron tunnelling in device grade ultra-thin 3–6 nm n + poly-Si/SiO 2 /n-Si structures has been analysed. The well known analytic expression for the Fowler-Nordheim tunnelling current was adapted to include the case of direct tunnelling of electrons, which becomes important for oxide layers thinner than 4.5 nm. For these ultra-thin oxide MOS structures it is necessary to take the band bending in the Si substrate and in the poly-Si layer into account to determine the oxide electrical field strength and to derive the tunnelling parameters of the measured current-voltage characteristic. A method is explained to derive the tunnel barrier height φ s and the effective mass of the tunnelling electron m ox from the experimental tunnel current characteristics. It is shown that both the direct tunnelling and the Fowler-Nordheim tunnelling current can be quantitatively explained by a WKB approximation using m ox as the single fitting parameter.

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

Variation in the fixed charge density of SiOx/ZrO2 gate dielectric stacks during postdeposition oxidation

The effect of postdeposition oxidation of SiOx/ZrO2 gate dielectric stacks at different temperatures (500–700 °C) on the density of fixed charge and interface states is investigated. It is shown that with increasing oxidation temperature the density of negative fixed charge is reduced, but the density of interface states increases. The net positive charge observed after oxidation at T>500 °C resembles the charge generated at the Si/SiO2 interface by hydrogen in the same temperatures range. This association is supported by the resistance of both types of charge against molecular hydrogen anneal but their fast removal in the presence of atomic hydrogen at 400 °C. Therefore, we propose that the observed oxidation-induced positive charge in the SiOx/ZrO2 gate stack may be related to overcoordinated oxygen centers induced by hydrogen.

Polarity effect on the temperature dependence of leakage current through HfO2/SiO2 gate dielectric stacks

A strong polarity effect on the temperature dependence of the leakage current in TiN/HfO2/SiO2/Si capacitors is reported. A model is proposed to explain these experimental results that combines tunneling through the stack and Frenkel–Poole hopping in the HfO2 layer, depending on the value of the gate voltage. It is shown that the polarity effect most probably results from the anisotropy of the band diagram of the HfO2/SiO2 stack, as well as from the location of the shallow traps with respect to the conduction band of the HfO2 layer. Comparison of the model with the experimental results allows an estimate of the trap depth to be between 0.5 and 0.8 eV.

Band alignments in metal-oxide-silicon structures with atomic-layer deposited Al2O3 and ZrO2

The energy barrier height Φ for electrons at the interfaces of various metals (Mg,Al,Ni,Cu,Au) with nanometer-thin Al2O3 and ZrO2 layers grown on (100)Si by atomic layer deposition has been directly measured using internal photoemission of electrons into the insulator. The behavior of the metal/Al2O3 contacts with increasing metal electronegativity XM resembles that of the metal/SiO2 interfaces with ideality factor dΦ/dXM≈1. The metal/ZrO2 contacts exhibit a less ideal behavior with dΦ/dXM≈0.75. The metal–silicon work function differences in structures with Al2O3 and ZrO2 insulators appear to be considerably larger than in the structures with thermally grown SiO2, suggesting the presence of a negative dipole layer at the metal/deposited oxide interface.

Undoped and in-situ B doped GeSn epitaxial growth on Ge by atmospheric pressure-chemical vapor deposition

In this letter, we propose an atmospheric pressure-chemical vapor deposition technique to grow metastable GeSn epitaxial layers on Ge. We report the growth of defect free fully strained undoped and in-situ B doped GeSn layers on Ge substrates with Sn contents up to 8%. Those metastable layers stay fully strained after 30 min anneal in N2 at 500 °C; Ge-Sn interdiffusion is seen at 500 °C but not at lower temperature. B is 100% active in the in-situ GeSn:B layers up to a concentration of 1.7 × 1019 cm−3. GeSn:B provides slightly lower Hall hole mobility values than in pure p-type Ge especially for low B concentrations.